1. Field of the Invention
The present invention relates to a system and method for switching between different antenna patterns to satisfy antenna gain requirements over a desired coverage angle. More particularly, the present invention relates to a system and method for switching between different antenna patterns used by a user terminal of a communications network based on the strength of a communications signal being received by the antenna of the user terminal, in order to satisfy desired gain and beamwidth requirements.
2. Description of the Related Art
A satellite-based communications network includes a plurality of user terminals which transmit and receive communications signals to and from, for example, a base station or other user terminals via one or more satellites, which can be low earth orbit (LEO) satellites, geosynchronous earth orbit (GEO) satellites, or a combination of both. Each user terminal includes an antenna that provides an antenna pattern which is suitable for transmitting and receiving the communications signals, which are typically radio frequency (RF) signals.
The types of antennas employed in user terminals generally provide a compromise between the antenna beamwidth and antenna gain. That is, as the antenna beamwidth broadens, the gain over the beamwidth drops. Therefore, certain antenna gain-to-noise temperature requirements (G/T) and beamwidth requirements are difficult to achieve with a conventional antenna.
For example, one network specification requires that the antenna gain should be 2 dBic over 75% of the solid angle. Assuming that the antenna beam is spherical, 75% of the solid angle translates into an angle of 150 degrees. This means that the gain of the antenna, which is assumed to be fixed, would need to be at least 2 dB over an angle of xc2x175 degrees from Zenith (or for elevation above 15 degrees). However, a current engineering model indicates that a gain of 2 dB can be sustained over an angle of only xc2x151 degrees (102 degrees total), which provides a beamwidth almost ⅓ narrower than the required beamwidth specification of 150 degrees. Furthermore, the G/T specification in this network is very stringent. Assuming a 2 dB antenna gain, the required noise figure of the receiver would need to be 2.36 dB for the entire receiver, which is impractical to achieve for high volume production of low cost mobile telephones. In order to achieve the required noise figure, it would be necessary to increase the gain of the antenna over the entire 150 angle, which cannot be achieved with a conventional fixed (non-swivel) antenna. It is noted that factors such as the restriction in size of the mechanical housing of the user terminal, electrical considerations such as losses, leakage and coupling, and power to the antenna, as well as economical considerations, can adversely affect the ability to increase antenna gain.
In an attempt to solve the above problems associated with fixed antennas, a user terminal can be modified to employ a swivel antenna to provide the required gain over the desired beamwidth. However, it is impractical to use a swivel antenna in a user terminal for a satellite-based communications networks employing satellites that are not geosynchronous. In such networks, the user terminal would continuously need to adjust the direction in which the antenna is pointing, because the satellites move with respect to the earth""s surface. This process becomes more complicated if the user terminal needs to simultaneously track two satellites orbiting the earth at two different elevations, especially if the user terminal is in a moving vehicle.
Accordingly, a need exists for an antenna that can be employed in a user terminal of a satellite-based communications network to provide enhanced gain and beamwidth features.
An object of the present invention is to provide an antenna arrangement that can be employed in a user terminal of a satellite-based communications network to provide enhanced gain and beamwidth features.
Another object of the invention is to provide an antenna arrangement, for use in a user terminal of a satellite-based communications network, that is capable of providing a stable, optimum gain over a wide coverage angle.
A further object of the invention it to provide a system and method that uses a single antenna configuration that can be controlled to provide different antenna patterns to satisfy antenna gain requirements over a desired coverage angle.
These and other objects are substantially achieved by providing a system and method for controlling an antenna arrangement comprising a plurality of antennas employed in, for example, a user terminal of a communications network, to provide a selected antenna pattern from among different antenna patterns based on, for example, the strength of a communications signal being received by the antenna arrangement of the user terminal, in order to satisfy desired gain and beamwidth requirements. The system and method can employ a switch that can be, for example, manually switched, or switched by a user controllable switch or automated arbitration process, to control the antenna arrangement to provide the selected antenna pattern. The switch also can include a plurality of diodes that are controlled automatically or, for example, by a user controllable switch or automated arbitration process, to select the antenna pattern. The plurality of antennas can be selectively activated and deactivated to provide the selected antenna pattern. Also, the plurality of antennas can be stacked vertically in relation to each other, or can include a first antenna and a second antenna disposed within said first antenna. Furthermore, the antenna arrangement can include at least one quadrifilar antenna or at least one octifilar antenna.